The technique uses interference patterns created when GPS signals that have reflected off the ground – called multi-path signals – are combined with signals that arrive directly from the satellite. Because the arrival of the multi-path signals at the GPS receiver is delayed, the signals have usually been viewed as noise, and ignored, according to Professor Kristine Larson of the University of Colorado at Boulder, who is leading the study.

However, using data collected from a field south of Boulder during two large snowstorms in March and April last year, the researchers were able to show that the modulation of the multi-path signals was directly related to the depth of the snow. Their GPS snow-depth estimates were strongly correlated with field measurements and nearby ultrasonic snow-depth sensors.

More recently, the team has been analysing how GPS signals travelling through alfalfa, corn and grass correlate with the moisture content of the vegetation.

‘We want to know if the water is on the ground, in the snow or in the vegetation, and how much is evaporating into the atmosphere, since it will ultimately be returned to the Earth’s surface through precipitation events,’ Larson said.

February 2010